This invention relates generally to apparatus for confining a plasma, and more particularly to a modular stellarator.
In the development of a device to confine a thermonuclear plasma, an obvious choice was a torus with coils to produce a toroidal field spaced about the torus. However, this was soon found unsatisfactory. Because of the geometry of the torus, the toroidal field lines curved around the toroidal axis such that the force associated with the magnetic field gradient caused positively and negatively charged plasma particles to drift in opposite directions. This charge separation resulted in an electric field which destroyed the magnetic confinement. Consequently, in order to prevent charge separation in a torus, a poloidal magnetic field must be applied. The two major toroidal confinement devices: tokamaks and stellarators differ in the manner in which the poloidal magnetic field is applied.
In a tokamak, the poloidal field is applied by introducing a toroidal current in the plasma. One drawback of the tokamak is that the large plasma current needed for confinement carries a large free energy that can be tapped by instabilities which destroy the confinement. Another drawback of the tokamak is the relatively small values of beta achievable--typically only up to 6%. The tokamak is also not a steady-state device; it only operates during pulsed operation.
Beta, .beta., is the ratio of plasma particle pressure to the magnetic field. High values of beta are important for achieving high plasma temperatures and confinement times. High betas are essential if a fusion reactor using only deuterium fuel is to be realized. Currently, deuterium/tritium is the fuel or choice because the D-T reaction requires lower temperatures and confinement times than the D--D reaction.
In a stellarator, the poloidal field is produced externally to the plasma, generally by current-carrying conductor wound helically around the torus. Stellarators with helically wound conductor and toroidal field coils are said to have interlocked coils because the helical conductor winds in and around the toroidal field coils. Stellarators are capable of achieving betas several times greater than in a tokamak and are also capable of steady-state operation. The major drawbacks of stellarators have been the complicated coil structure required and the lack of easy access to the inside of the machine for repair due to the helically wound conductor.
Recently, several modular stellarator designs have been proposed. In a modular stellarator, both toroidal and poloidal fields are achieved by a single set of non-interlocking non-planar coils. These designs generally include a circular (non-helical) axis and hence do not have the high beta or stability of the more complicated interlocked stellarators. Although the non-planar coils are modular and easily replaceable, they are difficult to manufacture.
Another version of a modular stellarator is built using only non-interlocked, circular coils on a helical axis. While this stellarator is easily maintainable and the coils easily manufactured, the most unattractive feature of this stellarator is that it has a maximum average B (magnetic field) at low .beta.--a magnetic hill. This condition implies instability for reactor grade plasmas.
It has been determined that stellarators which have magnetic wells--minimum average B at low .beta.--have good stability properties. The measure of a magnetic well is determined by the sign of V", the second derivative of volume per unit toroidal flux. Hence, when V"&lt;0, there is a magnetic well. The stability beta limit is .chi. 2/2A where .chi. is the rotational transform and A is the stellarator aspect ratio. (Rotational transform is the average twist or rotation of a magnetic field line in a magnetic surface. Aspect ratio is the ratio of the torus major radius to minor radius.) Hence a large rotational transform in the presence of a magnetic well gives a large beta.
Therefore, it is an object of the present invention to provide a modular stellarator using planar coils having a magnetic well and large rotational transform.
It is also an object of the present invention to provide a stable plasma confinement device using only toroidal field coils.
It is another object of the present invention to provide a stellarator that can be easily maintained, dismantled and repaired.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.